This invention relates to a method and apparatus for mixing a second material into a first fluid material by use of a centrifugal impeller. The second material is usually a dry particulate material but can also be a liquid or a gas, and the first material is normally a liquid.
Centrifugal pump mixers or blenders are well known in which dry ingredients, liquids, or gases are blended into a liquid by feeding the ingredients through a conduit into the inlet of a centrifugal pump having an open-face impeller. The liquid is supplied under pressure tangentially into an annular passageway surrounding the conduit and then directed outward for flow through at least a portion of the impeller passageways before discharge from the pump casing. Some mixing of the liquid and the added ingredients occurs in the pump impeller, but such mixing is limited because the liquid does not flow inward to join with the added ingredient in the center of the impeller but instead is diverted by a flange at the end of the conduit to flow outward between the pump casing and the open-face centrifugal impeller. The pump casings in these prior art mixers are of conventional volute form and have a single outlet tangent to the outer circumference. In a typical process application, the liquid carrying the entrained added ingredients will be conducted through an external pipeline to a separate storage tank. Normally, more thorough mixing is required than results from a single pass through the mixing pump; so a line will be connected between the bottom of the storage tank and the inlet to the annular passageway for recirculating a portion of the mixture in the tank back through the mixing pump.
Except for mixtures of very low viscosity, such a system will require a second centrifugal pump in the recirculating supply line for mixtures of moderate viscosity (up to approximately 20% solids). For high viscosity mixtures, a third support or booster pump may be needed in the discharge line between the mixer and the storage tank and both the supply and the discharge recirculating booster pumps may have to be of positive displacement type for the thicker mixtures (40 - 60% solids). The recirculation support pumps, pipe and fittings required for operation of these prior art mixers not only add significantly to the cost of a complete mixing installation but also create a cleanout problem, particularly if recirculation is stopped and the mixture allowed to set up in the system.